1. Field of the Invention
The present invention relates to optical semiconductor devices. In particular, it relates to high efficiency optical semiconductor devices used for rewritable optical discs and high-speed, large-capacity optical communications.
2. Description of Related Art
As we are in a sophisticated information society, high-speed/large-capacity optical communication technologies have been demanded for communication tools typified by the Internet. At the same time, optical discs which are rewritable at higher speed with larger capacity have been demanded as a means for storing large volumes of information obtained through the communication. Under these circumstances, optical semiconductor devices such as semiconductor lasers are assumed as key devices in the optical communication and optical disc technologies. Therefore, improvement in performance, functionality and reliability of the optical semiconductor devices has been longed for.
A major technique for enhancing the performance of the optical semiconductor devices is to connect a semiconductor laser chip to a base. As an example, FIG. 10 shows the structure of a conventional optical semiconductor device.
FIG. 10 shows a major part of an optical semiconductor device 10 used as a light source for optical communications. The optical semiconductor device 10 includes a Si substrate 3 and a distributed feedback semiconductor laser chip 1. A SiO2 film 5 is formed on the top surface of the Si substrate 3 and an electrode pattern 6 is formed on the SiO2 film 5. Further, a solder layer 7 is formed on the electrode pattern 6 so that the semiconductor laser chip 1 is adhered to the electrode pattern 6 via the solder layer 7.
The semiconductor laser chip 1 has a mesa portion 8 formed in the middle of the bottom surface thereof. The solder layer 7 is applied to part of the bottom surface of the semiconductor laser chip 1 except the mesa portion 8. That is, a gap 9 is formed between the mesa portion 8 and the SiO2 film 5. Further, an active layer 2 for emitting a laser beam is formed in part of the mesa portion 8 near the bottom surface of the semiconductor laser chip 1.
In this optical semiconductor device 10, the semiconductor laser chip 1 is mounted on the Si substrate 3 in the following manner. First, molten solder is applied onto the electrode pattern 6 and the semiconductor laser chip 1 is laid on the molten solder, followed by cooling to solidify the solder. In general, the semiconductor laser chip and the Si substrate have different thermal expansion coefficients. Therefore, during the solidification of the solder, the active layer 2 may be distorted or residual stress may be caused in the active layer 2.
However, since the gap is formed between the mesa portion 8 and the SiO2 film 5 as described above, the active layer 2 does not contact the solder layer 7. Therefore, the possibility that the active layer 2 is distorted or the residual stress occurs in the active layer 2 is significantly reduced. As a result, the distributed feedback optical semiconductor device 10 becomes capable of operating in a single mode at a stable oscillation wavelength (e.g., see Japanese Unexamined Patent Publication No. 2002-314184).
Another optical semiconductor device according to Japanese Unexamined Patent Publication No. H11-87849 (not shown) has substantially the same structure as the optical semiconductor device 10 except that the mesa portion is not formed in the semiconductor laser chip. Also in this case, the semiconductor laser chip and the substrate for mounting the semiconductor laser chip thereon have different thermal expansion coefficients. Therefore, residual stress occurs in the active layer of the semiconductor laser chip after it is mounted on the substrate. The residual stress may possibly cause instability in characteristics of a diffraction grating of the active layer. However, as the gap reduces the possibility of the instability in the characteristics of the diffraction grating of the active layer, stable oscillation characteristics are obtained.
Further, Japanese Unexamined Patent Publication No. 2003-23200 discloses a structure intended to reduce the residual stress, improve laser characteristics when the semiconductor laser chip is operated at a high temperature and improve the reliability of the semiconductor device. Specifically, a groove is formed in part of the top surface of the substrate opposing the mesa portion and the solder layer is formed in the groove in a stripe form. As the solder layer does not contact part of the bottom surface of the semiconductor laser chip around the mesa portion, the residual stress is reduced. Further, in part of the bottom surface of the semiconductor laser chip outside the mesa portion, another solder layer having a higher melting point than that of the stripe-shaped solder layer is provided substantially parallel to the groove. With this structure, the semiconductor laser chip is electrically connected to the substrate, while the excellent ability of dissipating heat of the semiconductor laser device is ensured.